Hello, welcome to Healthcaremagic. Ensure is an energy drink which supplements proteins and other nutrients along with calories. I dont see any harmfull effect of it in cancer patients infact it is beneficial as in chemotherapy and radiation recieving patients the demand of nutrients in body increases because of regenration needs of body cells. So you can continue using it in my view without any apprehension. Still it would be in your favour if would consult this with your treating doctor as well. Wish you good health.